Optical memory disks, such as CD (compact disks), CD-R, CD-RW; DVD (digital versatile disks), DVD-R, DVD-ROM, DVD-RAM, DVD+RW, DVD-RW, PD (phase change disks) and MO (magneto optical), etc., are typically manufactured by initially forming a substrate and then depositing one or more thin film layers upon the substrate. Substrates for optical memory are usually formed with a series of grooves and/or pits arranged as concentric tracks or as a continuous spiral. The grooves and pits may be used for things such as laser beam tracking, address information, timing, error correction, data, etc. Substrates used for optical disks are typically formed by injection molding, where a molten polymeric material is injected into a disk shaped mold with one surface having the patterned microstructure to be replicated. The patterned microstructure is typically provided by an exchangeable insert, commonly referred to as a stamper. The injection molding process is comprised of a series of precisely timed steps, which include closing the mold, injecting the molten polymer, providing a controlled reduction in peak injection pressure, cooling, center-hole formation, opening the mold and removing the replicated disk and associated sprue. Following the molding process, disk substrates are typically coated with one or more thin film layers. Thereafter, substrates may be coated with various insulating and/or protective layers, bonding adhesive, decorative artwork, labels, etc.
Although injection-molding methods, such as those described above, can provide high quality optical memory disks with acceptable levels of birefringence and flatness, the rate of disk production is only in the neighborhood of several seconds. About 60% of this time is attributable to the molding step, and the rest is taken up by the need to open the mold, remove the disk and sprue, and then close the mold before the next cycle can begin. Furthermore, present attempts to improve production rate by using various novel de-molding techniques or by using multi-cavity molds have had only limited success.
Besides lower than desired production rates, injection molding requires complex closed-loop control over numerous parameters. For example, mold and polymer temperature, press clamp force, injection profile and hold time all have competing and often-opposed influences on birefringence, flatness, and on the accuracy of the replicated features. It should also be noted that molding difficulty increases as the thickness of the replicated disk decreases. So where standard CD substrates, which are approximately 1.2 mm thick, do not require the use of specialized techniques, such as increasing the molding cavity cross-section during the main injection phase, injection-compression molding, coining, “bump molding”, etc., standard DVD substrates, which are approximately 0.6 mm thick, do in order to simultaneously meet birefringence and flatness specifications.
The trend in future optical memory products is toward thinner substrates and/or smaller disks. Directly manufacturing these products via injection molding may not be practical. For small diameter disks (i.e. 5–8 cm.), such as the ones used in Personal Digital Assistants (PDA's) and Digital Electronic Cameras, disturbances caused by center gating can influence the quality of the innermost tracks on the disk. These disturbances are associated with local turbulence, shear, and packing variation near the center gate in the mold and can produce locally poor flatness and high birefringence. As the minimum track diameter is reduced, these problems may be exemplified.
To speed-up the rate of manufacturing, a number of methods for manufacturing optical memory using continuous web processes have been proposed. These methods are built on the concept of forming a microstructure pattern on a continuous web of material by passing the web between a roller and a stamper.
To date, there have been two types of continuous web processes proposed. These processes include “in-line” and “off-line” methods. In-line continuous web processes integrate web extrusion with microstructure pattern formation in the same process, while off-line continuous web processes carry out web formation on pre-fabricated web material which is manufactured on another production line. The goal of in-line formation is to contact the web with a stamper immediately after web extrusion and while the web is still hot. Examples of in-line processes include those described in U.S. Pat. Nos. 5,137,661; 4,790,893; 5,433,897; 5,368,789; 5,281,371; 5,460,766; 5,147,592; and 5,075,060, the disclosures of which are herein incorporated by reference. The integration of web extrusion and web formation requires that a disk manufacturer not only engage in the business of producing optical disks but also in web extrusion. This makes the overall system a highly complex process, at a point in the process where it may not be desirable. Furthermore, because the disk manufacturer may not enjoy the same economies of scale that a plastic web manufacturer does, the cost per unit for disks formed with in-line processes may be higher than that for off-line processes. Thus, the present inventors propose that off-line processing not only offers the opportunity for improved throughput, reduced cost and complexity, and shorter start-up time, but for increased process flexibility as well.
One method of web formation, Which may be used for in-line processes for optical memory production, is proposed by Kime, U.S. Pat. No. 6,007,888, entitled “Directed Energy Assisted In Vacuo Micro Embossing” which issued Dec. 28, 1999, the disclosure of which is herein incorporated by reference. Kime discloses a continuous manufacturing process using directed energy assisted micro embossing. The patent describes a directed energy source used to heat web material and a stamper before they are pressed together by a pair of nip rollers.
Although Kime is well regarded for what it teaches, when increasingly higher density data devices are formed, a number of factors not normally at issue arise. For example, the preset inventors have found that unavoidable variation in web surface texture and web thickness exist and can interfere with fine microstructure reproduction. These variations result in locally, non-uniform contact pressure between the web and stamper. In a process where the web is softened to form the microstructures, simply increasing the average contact pressure fails to adequately solve this problem, as excessively high contact pressure may result in a distorted image of the surface due to elastic rebound within the web material after pressure is removed. Stamper web relative movement can also cause ‘smearing’. Smearing distorts the shape of the data tracks and/or pits on a microscopic scale. These distortions can interfere with tracking and can also increase read-back error rates. Accordingly, there is a need for a method and/or apparatus, which accommodates the negative effects produced by variations in web surface texture and web thickness.
In order to accurately replicate stamper microstructure, many have tried to keep the stamper in contact with the web long enough for the displaced polymer to relax and the substrate cool. However, it was found by the present inventors that simply increasing contact time is not an acceptable solution due to the resultant increase in warp. As may be appreciated, a warped disk produces significant read problems. Warp related problems become even a greater problem with writeable disks, where the quality of the recording can be degraded and compounds the detrimental influence of warp during read-back. Accordingly, there is a need for a continuous method for producing optical memory and/or apparatus which limits warp during substrate formation.